Many electric machines, such as electric motors and electric generators, include a stator that is held stationary and a rotor that rotates adjacent the stator. The stator and rotor may be configured to transfer power between one another through one or more rotating magnetic fields. Some electric machines may include a permanent-magnet type rotor with permanent magnets mounted on or inside a body of the rotor to create a magnetic field that rotates with the rotor. Each permanent magnet of the rotor may individually create a north or south magnetic pole of the rotor. A permanent-magnet type rotor having only a single permanent magnet creating each of its magnetic poles may, however, limit the performance potential of the associated electric machine.
U.S. Pat. No. 6,555,940 to Naito et al. (“the '940 patent”) shows a permanent-magnet rotor having a plurality of permanent magnets creating each magnetic pole of the rotor. The permanent-magnet rotor of the '940 patent includes five arcuate permanent magnets mounted in each quadrant of a rotor core with the concave sides of the arcuate permanent magnets facing radially outward with respect to a central axis of the permanent-magnet rotor. In two of the quadrants of the rotor core, the north magnetic pole of each permanent magnet faces away from the central axis of the rotor, such that the five permanent magnets create a north magnetic pole. In the other two quadrants, the south magnetic poles of the permanent magnets face away from the central axis of the rotor, such that the permanent magnets disposed in each of those quadrants create a south magnetic pole of the rotor.
The five permanent magnets in each quadrant of the rotor core of the '940 patent are arranged in three radial layers with respect to the central axis of the rotor. An outer radial layer includes one of the permanent magnets with its convex side facing radially inward. An intermediate layer includes two of the permanent magnets extending along a common arc parallel to, and radially inside of, the first permanent magnet. An inner layer includes the remaining two magnets extending along a common arc parallel to, and radially inside of, the two permanent magnets of the intermediate layer. In each quadrant, portions of the rotor core disposed between the layers of permanent magnets are connected to portions of the rotor core disposed radially inside of the inner layer of permanent magnets only by thin portions of the rotor core extending past ends of the permanent magnets in the inner layer.
Although the permanent-magnet rotor of the '940 patent includes multiple permanent magnets that create each of its magnetic poles, certain disadvantages persist. For example, portions of the rotor core between the layers of permanent magnets have relatively large masses, which may cause relatively large centrifugal forces on these portions of the rotor core during rotation of the rotor. This may create undesirably high stresses in the relatively narrow portions of the rotor core that connect the portions of the rotor core between the layers of magnets to the portions of the rotor core disposed radially inside the inner layer of magnets. Additionally, during use of the rotor of the '940 patent in an electric motor or generator, the temperature of permanent magnets of the rotor and the portions of the rotor core disposed between adjacent layers of permanent magnets may increase. As the temperature of the portions of rotor core between adjacent layers of the permanent magnets increases, these portions of the rotor core may accept less heat from, or even reject heat to, the adjacent permanent magnets. This may cause the permanent magnets to reach undesirably high temperatures.
The electric machine of the present disclosure solves one or more of the problems set forth above.